Step 1: Materials and Supplies
Stretchy denim- I found a remnant that I believe was approximately .44 yards. You want your fabric to be 3-4 inches longer than your baby's waist-to-foot height.
Elastic- enough to go around your baby's waist
Pants that fit- it's easiest to make your jeggings using a pair of pants that currently fits your baby well
Step 2: Cutting Your Pieces
Before doing anything with your pants, pull your denim material in both directions to find which way is stretchier. You want the pants to stretch from side to side, not from top to bottom. Arrange the material so that you can stretch it the most horizontally.
Choose one piece of the pants you're working with to follow as a guide and pin that piece to your denim as flatly and evenly as possible. I began with one of the front pieces. If you have to tuck and fold fabric from the other pieces on the pants to get the piece you're working with to lay flat, that's okay.
Cut the denim around the pinned panel, leaving approximately 1/2 inch of space for seams. When you get to the crotch and middle of the pants, fold them over so the pinned section is still laying flat, but you should be able to see where to cut (see photo 3). Cut the waist at the bottom edge of the pant waist. You'll add a waistband later.
Once you have one front panel cut out, the easiest way to make the other symmetrical is to pin the cut piece to the fabric, right sides together, and cut around both pieces. Repeat the pinning and cutting steps with the back of the pants, and you should have four panels (if using the same style of pants a I did) that somewhat resemble jean legs.
Cut out two strips of fabric, approximately 1.5 inches tall and at least long enough to go across the waist of the pants (be sure the fabric is stretchiest going around the waist). Set aside for the waistband. If you're adding back pockets, cut out two pocket shapes slightly larger than you'd like them to be on the finished jeggings and set aside.
Step 3: Sew
Open the two pieces of fabric you just sewed and pin right sides together beginning at the waist going towards the crotch. Pin through the crotch and turn to pin towards the waist on the other side (see photo 4). This will become the front, back and crotch seam. Sew along the pinned edge in one continuous seam, making sure not to leave any open areas where the four panels intersect.
You should now have all four panels sewn together. Line up the front seam with the back seam, right sides together, and you should see your jeggings begin to take on pant form! Pin and sew along the outer edges to make your outer leg seams.
Step 4: Add a Waistband and Hem
With your jeggings turned right side out, fold the waistband material in half, making a casing for the elastic. Pin to the outside upper edge of the jeggings with all unfinished edges facing up (see photo) and matching outer seams of the jeggings to the seams on the waistband. Sew around the waist, leaving a gap of approximately one inch to insert the elastic.
Thread the elastic through the opening and around the waist of the jeggings. Pull both ends through and sew together, making one loop of elastic for the waist. Trim excess elastic. Tuck elastic into the waistband opening and sew opening shut.
Turn the jeggings inside out to hem the ankles. Fold the bottom edge of the pant legs over approximately 1/4-1/2 inch and sew in place on both legs. Turn jeggings right side out.